This invention relates to the generation of charged particles, and more particularly, to the generation of ions with high current densities.
Ions can be generated in a wide variety of ways. Common techniques include the use of air gap breakdown, corona discharges and spark discharges. Other techniques employ triboelectricity, radiation (Alpha, Beta, and Gamma, as well as x-rays and ultra-violet light) and microwave breakdown.
Air gap breakdown, i.e., discharges occurring in small gaps between a stylus or wire and the surface of a dielectric material, are widely employed in the formulation of electrostatic images. Representative U.S. Pat. Nos. are G. R. Mott 3,208,076; E. W. Marshall 3,631,509; A. D. Brown, Jr. 3,662,396; A. E. Bliss et al. 3,792,495; R. F. Borelli 3,958,251; and R. T. Lamb 3,725,950.
In the case of an air gap breakdown, it is necessary that the gap spacing be maintained between about 0.0002 and 0.0008 inches in order to be able to operate with applied potentials at reasonable levels and maintain charge image integrity. Even then, the latent charge image is not uniform, so that the resultant electrostatically toned image lacks good definition and dot fill.
An alternative to air gap between is the corona discharge from a small diameter wire or a point source. Illustrative U.S. Pat. Nos. are P. Lee 3,358,289; Lee F. Frank 3,611,414; A. E. Jvirblis 3,623,123; H. Bresnik 3,765,027; P. J. Magill et al. 3,715,762; and R. A. Fotland 3,961,574. Corona discharges are widely employed in electrostatic precipitation, and are used almost exclusively in electrostatic copiers to charge photoconductive surface prior to exposure. Corona discharges are also extensively employed in electrostatic separators and in electrostatic coating and spraying equipment.
Unfortunately, standard corona discharges provide limited currents. The maximum discharge current density heretofore obtained has been on the order of 10 microamperes per square centimeter. This can impose a severe printing speed limitation. In addition, coronas can create significant maintenance problems. Corona wires are small and fragile and easily broken. Because of their high operating potentials, they collect dirt and dust and must be frequently cleaned or replaced.
An alternative technique for forming high density corona discharges is to use high velocity air streams. For example, if high pressure air is employed with a small orifice at the corona discharge point, current densities as high as 1000 microamperes per square centimeter are reportedly obtainable (Proceedings of the Conference on Static Electrification, London 1967, Page 139 of The Institute of Physics and Physical Society, London SW1). This technique is awkward, however, and requires both a pressurized air source and critical geometry in order to prevent premature electrical breakdown.
Another method of forming ions, which is particularly useful in electrostatic applications, uses an electrical spark discharge. Representative U.S. Pat. Nos. are B. E. Byrd 3,321,768; H. Epstein 3,335,322; C. D. Hendricks, Jr. 3,545,374; and W. P. Foster 3,362,325. A low energy spark discharge technique is described by Krekow and Schram in IEEE transactions on Electronic Devices, E.D.-21 #3, Page 189, March, 1974. The electrical spark discharge is objectionable, however, where uniform ion currents are desired or required. This is particularly true where the discharge occurs over the surface of a dielectric.
Accordingly, it is an object of the invention to facilitate the generation of ions, particularly at high current densities.
Another object is to provide a reliable and stable source of ions. A related object is to provide an ion generating system which does not require critical periodic maintenance. Another related object is to simplify maintenance and eliminate the objectionable characteristics of corona wires, including the fragility and tendency to collect dirt and dust.
A further object of the invention is to provide an easily controlled source of ions. A related object is to provide a multiplexable source of ions using different voltage sources to supply an alternating breakdown field and an ion extraction field.
Yet another object of the invention is to generate ion currents for use in producing electrostatic images in which charge image integrity is maintained. A related object is to achieve comparatively uniform charge images which can be toned with good definition and dot fill.
Further objects are to achieve increased electrostatic printing speed; suitable charge densities without requiring a pressurized air source and critical electrode geometry; and uniform ion density.